Currently, 3rd generation (3G) cellular communication systems are being developed to further enhance the communication services provided to mobile phone UEs (user equipment). The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology, namely Wideband Code Division Multiple Access (WCDMA). Carrier frequencies are used for uplink transmissions, i.e. transmissions from a mobile wireless communication unit (often referred to as wireless subscriber communication unit or user equipment in 3rd generation systems) to the communication infrastructure via a wireless serving base station (referred to as a Node B in 3rd generation systems) and downlink transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a wireless serving base station (e.g. Node B). A further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of Universal Mobile Telecommunication System (UMTS™), can be found in ‘WCDMA for UMTS’, Harris Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
CDMA communication, as used in 3G mobile communications air interface technologies, is an ‘interference limited’ technology from a data throughput perspective. CDMA technology utilises orthogonal variable spreading factor (OVSF) codes combined with pseudo noise (Pn) codes to differentiate multiple UEs that are utilising the same spectrum at the same time for uplink access on the Uu radio interface. In order to maintain sufficient signal-to-interference ratio (SIR) protection for all UEs on accessing the Node B, up-link (UL) power control (PC) is dynamically managed by the network infrastructure. SIR estimation is commonly derived from pilot tones in the uplink (UL) dedicated physical control channel (DPCCH). User equipment (UE) devices transmitting to a Node B on the same spreading factor (SF) code would be arranged such that their respective transmissions have substantially the same power when received at the receiving node B. Often, up to ninety six UEs are simultaneously supported in call mode for a specific Node B.
CDMA technologies are known to have a relatively robust performance to multi-path effects. CDMA uses the multi-path signals and combines them to make an even stronger signal to be received at, and processed by, the respective receivers. An example of a known CDMA wireless receiver 100 is illustrated in FIG. 1. The CDMA wireless receiver 100 comprises an antenna 102 receiving a radiated signal and providing the received radiated signal 104 to radio frequency (RF) circuitry 106. The RF circuitry 106 provides a frequency down-converted and filtered signal 108 to an analog-to-digital converter (ADC) 110, which in turn provides a digital representation 112 of the received signal to a RAKE receiver 114. CDMA receivers use rake receivers, which is essentially a set of several digital parallel-configured versions of the received signal often referred to as rake fingers. The RAKE receiver 114 comprises receive filter 116 arranged to filter the digital representation 112 and provide the filtered digital signal 188 to a RAKE finger bank 118 and path selection logic 120. The path selection logic 120 is, in essence, one of the rake receivers and is arranged to constantly search for different multi-path signals. The path selection logic 120 feeds the information, via RAKE control signal 122, to the other rake fingers in the RAKE finger bank 118. Each rake finger of the RAKE finger bank 118 then selects the signal corresponding to a recursively strongest multi-path, and provides an output received signal 124, as selected by the RAKE control signal 122, to a maximum ratio combiner 126 to combine the respective RAKE received signals to make the composite signal stronger. The RAKE finger bank 118 also provides a signal 198, which is an estimate of a selected RAKE path performance.
In a WCDMA system, as deployed for 3G, signal 198 could be the signal-to-interference ratio (SIR) estimate from the pilot tone recovered in the dedicated physical control channel (DPCCH) or could be estimated within the dedicated physical data channel (DPDCH) being processed. This signal is used by channel estimation logic 130, which provides a channel estimation signal 132 to the maximum ratio combiner 126. The maximum ratio combiner 126 is able to combine appropriate RAKE received signals under control of the channel estimation signal 132 and provide a suitable RAKE received signal 134 to channel codec logic 136 for decoding. The decoded RAKE received signal is then output 140 from the RAKE receiver 114.
Conventional antenna arrays, comprising multiple antenna elements and used with existing Node B equipment in most 3G installations, utilise a fixed +/−65° beam pattern. Outside of the main lobe of the antenna beam the signals are spatially filtered and significantly attenuated. Conventional network planning and passive antenna array solutions process all incoming signals with a common fixed beam pattern. This receive processing based on signals received within the geographic area identified by the antenna main lobe, tends to dictate a corresponding common beam pattern for transmitter operation. Thus, an identical radio frequency (RF) footprint is used for both receive (Rx) and transmit (Tx) operation.
Rx beam-forming using antenna arrays depends on the ability to constructively add incident signals on each of the antenna elements in a way that coherently adds those from the desired direction. Thus, incident signals that are not from the desired direction will be incoherently added, and thus will not experience the same processing gain. The term ‘coherency’ implies that the signals will have substantially the same phase angle when added beam forming process. In addition, thermal noise from multiple sources also exhibits incoherent properties, and thus when added the signals from multiple sources do not experience the same processing gain as a coherent desired signal.
Conversely, in transmit active antenna arrays, the signals are coherently combined within the intended beam pattern as electromagnetic (EM) signals in the ‘air’ so that they arrive coherently at the mobile station (MS) (e.g. UE) receiver.
In a Node B antenna array, the received RF signal from a single UE cannot be discerned without demodulation of the composite signal. Individual receive beam-forming for a specific user is not feasible, since there is likely to be multiple received signals of the same power from different UEs simultaneously at the antenna array. Even if few UEs are utilising the Node B, the likelihood is that the signals would be below the noise floor of the Node B's receiver. The processing gain of the WCDMA receiver implies that the signal can be extracted from the noise floor. This, however, requires at least a partial demodulation process.
An example of a known cellular network plan 275 is also illustrated. The Cellular network plan utilises cells 280 in a “honeycomb” cellular structure. Cells are divided into sectors 285. Typically, three sectors exist per cell corresponding to approximately 120° coverage per sector. One sector corresponds to a radiation pattern of a single conventional antenna array whose beam pattern of +/−65° maximally covers the sector. Six sector cells, though less common, are utilised in some network configurations. CDMA and WCDMA technologies are able to use a single modulated RF carrier frequency for all uplink users on all cells and sectors within those cells. Likewise, a single carrier frequency is used for downlink on all cells in the network and all sectors within these cells. Furthermore, each sector antenna RF signal is processed independently by its respective receiver or transmitter.
To those skilled in the art it will be known that a cellular hierarchy consists of a macro cell for wide area geographic coverage and micro, pico and femto cells used for small localised coverage within the macro geographic structure. In some cases the micro, pico and femto cells may use the same frequency as that of the macro cell. Since the micro, pico and femto cells are used for in-fill coverage, their radiative antenna units will not be sharing the same physical site as the antennae used in the macro sector.
A WCDMA receiver is illustrated in Chapter 5 of the book, titled: “Smart Antennas: State of the Art” (Eurasip Book Series on Signal Processing & Communications), edited by Kaiser et al ISBN-10: 9775945097 and authored by Klemens Freudenthaler et al. Chapter 5 deals with maximum ratio combining (MRC) space time Rake receiver, and considers independent Receivers from independent antenna combined through an MRC implementation. There is no beam-forming prior to the rake receivers. As a consequence, this proposal provides no spatial filtering to the RAKE receiver and is at a disadvantage from a noise perspective as the processing gain from the beam former is eliminated. Noise is suppressed in beam-forming by virtue of being uncorrelated. Conversely, the wanted received signal has correlative properties at the combining stage of the beam-former. In addition, this system also suffers from the feedback effect of convergence on to the strongest received WCDMA signal. This convergence time, particularly when multi-path signals change abruptly, can cause impaired link performance.
Referring now to FIG. 2, a known antenna array beam-forming arrangement 200 for a CDMA receiver is illustrated. The antenna array beam-forming arrangement 200 comprises multiple antennas 205, 215 coupling received signals to receivers 210, 220 comprising radio frequency (RF) receiver front-end circuits arranged to down convert received RF signals to baseband, analogue-to-digital converters arranged to convert the down-converted signals to a digital form and digitally filtered using a Root of Raised Cosine (RRC) matched channel filter arranged to extract the received signal. Each respective output from the RRC filter is input to beam-forming-space-time RAKE receiver function 225.
The beam-forming-space-time RAKE receiver function 225 comprises a spatial combining function comprising a delay estimate 235 arranged to estimate respective delays of the received signal at each antenna of the antenna array. The output of the delay estimate 235 is then input to a function that calculates a plurality of spatial weight vectors to be applied to beam-forming logic 245, 250, each of which additionally receive the RRC filtered output from their respective receiver. Following convergence, the beam-forming logic 245, 250 outputs a signal whose SIR is maximised. This signal is further processed in a multi-path (temporal) combining function 255 that comprises a RAKE receiver finger bank 260 receiving the plurality of beam-form corrected signals. The RAKE receiver finger bank outputs are input to a multipath combiner 265 where they are constructively combined and input to the usual channel decoding 270 process, etc.
However, a known problem associated with such processing of the received strongest multi-path signal is that the multi-path signals may change quickly due to only a small movement in a UE's location, angular position or motional objects within the signal scattering environment. As a consequence, antenna beam-forming techniques, where the beam has a relatively narrow beam, are rarely used in CDMA systems. This is due to the fact that a communication link is prone to being lost when the multi-path effect changes, and the use of narrow-beam, beam-forming antenna arrays exacerbates this effect. Thus, in known systems, the use of antenna array beam-forming has not been as widely adopted as it may have been, as dedicating UEs to single narrow beams has drawbacks in a mobile environment that has to accommodate multi-path signals. For example, in certain communication scenarios such as communications in urban environments, an undesirable, widely-scattered, received signal may result from a use of narrow-tracked antenna array beam-forming. To extract the full benefit from such a widely-scattered, received signal, narrow beam processing is ill advised as the complete multi-path combining effects of using rake receivers is made redundant and degraded performance ensues.
Furthermore, in conventional antenna systems for cellular communications, the transmit/receive beam pattern is often controllable using electromechanical beam steering elements, such as mechanical phase shifters. Thus, beam steering is currently performed remotely by manipulation of electromechanical elements of an antenna array located at the top of the antenna mast. Electromechanical beam manipulation is limited to minor changes in tilt (typically up to 10°). Electromechanical phase shift beam manipulation does not extensively change the main beam shape.
In order to address the limited flexibility associated with electromechanical phase shifter beam manipulation on an antenna array located at the top of the antenna mast, there has been recent interest in the use of smart or active antenna technology. This is a radio technology where the antenna system has dedicated signal processing per antenna array element or co-located antenna and signal processing units. The active antenna technologies fall into three broad families.
(i) multi-antenna systems (MAS);
(ii) radiohead with or without multiple in-multiple out (MIMO) signals; and
(iii) radio-array.
It is known that active array antenna technology is able to facilitate independent and variable beam patterns in both directions.
Modern air interface protocols, such as WCDMA, allow multiple UEs to simultaneously transmit to the base-station on the uplink on a single carrier frequency. A minimal limit of signal to noise per bit (Eb/No) ratio is required on the uplink channel to ensure adequate bit error rate (BER). This implies that the higher the data rate for a particular UE at least a proportionally better Carrier to interference plus noise ratio (CINR) is required to maintain Eb/No.
As the cell becomes more loaded, all UEs have to compensate by increasing transmit power so as to maintain Eb/No at the Node B receiver, since other UEs will add to the interference level. Likewise UEs requiring higher data rate services will require all UEs to compensate for higher Eb/No required of the Node B. This increase in transmit power by the UE can be detrimental to network performance. The increased power levels in the cell propagate to neighbouring cells forcing that cell to increase power levels to maintain Eb/No even though it may not be loaded heavily.
This effect proposes a number of problems to WCDMA networks. For example, the noise floor of the communication cell is increased, which in turn causes a propagation effect on the network and reduces data transmission rates. Furthermore, UE devices consequently consume more power, as Uplink transmissions are required to operate at higher power levels. In addition, limitations exist on maximum throughput per cell as a result of dynamic range limitations imposed on UE transmissions.
A further enhancement to the above proposed techniques is provided in the paper, titled: “Maximum ratio combining of correlated diversity branches with imperfect channel state information and colored noise”, authored by Schmitt et al. and published in ‘Spread Spectrum Techniques and Applications, 2004 IEEE Eighth International Symposium 30 Aug.-2 Sep. 2004, pages 859-863. Schmitt et al consider multiple active beam-forming branches that are MRC combined and show the benefits of up to 6 dB Eb/No for processing multiple beams. Unfortunately, the implementation proposed by Schmitt et al., for use in a node B would increase the amount of complexity in signal processing four-fold as each receiver path would require a full complement of dedicated Rake receivers.
Schmitt et al. also propose an arrangement that only supports a plurality of narrow beams. This can be to the detriment of propagation cases where there are wide scattering properties, and, thus, full coverage of the cell sector is undermined.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting active antenna array technology in a wireless communication network, for example in utilising and processing multi-beam capability on an up-link communication channel to improve air interface (Uu) link performance, would be advantageous.